Traditional vehicle transmissions utilizing gears are generally limited to a fixed number of gear ratios. The discrete steps associated with gear changes do not allow for optimal power transfer between the vehicle's engine and the wheels. The torque of an engine is usually constant while driving torque needs vary with speed and load on the engine. Higher torque is applied to driving axles at low speeds, and lower torque is usually applied at higher speeds. Some prior art transmission systems attempt to replace discrete gear transmissions having fixed input/output ratios with various friction, continuous drive arrangements. There are several known types of frictional continuously variable transmissions (CVTs). (Other types of CVTs include electrical CVTs, hydraulic CVTs, and planetary gear systems, all of which are outside of the scope of this disclosure.) One frictional CVT system is a pulley-based system, such as that claimed in U.S. Pat. No. 7,044,873, utilizing two pulleys with inversely variable diameters and a v-belt. The v-belt is kept under tension by pulleys and transfers rotary motion between the pulleys. Another type of prior art system is a toroidal CVT, which utilizes rotating toroidal members and disc rollers. The disc rollers contact the toroidal members thus transferring the rotational momentum between them. Examples of this type of design are offered in U.S. Pat. Nos. 2,164,504 and 7,077,780.
Yet another common type is a so-called ‘cone-and-idler’ system, schematically presented in FIG. 1 in a dual-cone configuration. The ‘cone-and-idler’ system comprises a driving cone, a driven cone, and an idler transferring the torque between them. In a conventional ‘cone-and-idler’ system, driving and driven shafts carry rotationally symmetric and typically equivalent driving and driven cones that are connected to the engine and to the drive axle, respectively. The idler, comprising a wheel on an idler shaft, is disposed between the cones, and the wheel is normal to the cones' outer surfaces and repositionable along the length of the cones while frictionally transmitting torque to effect a ratio change of driving to driven shafts.
Most continuously variable ‘cone-and-idler’ frictional transmissions require a high level of complexity in torque transmission and control actuation. In the three-body prior art configuration of FIG. 1, for example, mutual positioning of the driving and the driven conical members must assure a constant gap width between the cones. This requirement, in the case of the equivalent cones, imposes a restriction on the position of the respective rotatable shafts, which must be strictly parallel to one another. The idling wheel is typically “wedged” in the gap between the cones. The idling wheel transmits rotation and is movable within the gap along the conical surfaces to change the speed ratio. This configuration offers a wide speed ratio range. However, the overall design is rather complex because measures must be taken to maintain the proper gap width between the cones to accommodate friction among the cones and the idler. Further, the idler and cone-member structures are entangled, which does not allow easy access to and maintenance of the transmission. For example, to merely replace a worn out idler wheel, the entire transmission, including the cone facility, must be dismantled.
Another shortcoming in some of the prior art cone-and-idler traction transmissions is that they utilize multiple torque paths (such as the transmission described in U.S. Pat. No. 4,459,868.) In the process of varying the input/output ratio, all torque paths should be identical in size. If the torque paths are not equivalent, one idler torque path may cause a greater output than the other paths, causing slippage of the idler(s) with respect to the cone(s). A simple version of a cone-and-idler CVT could, therefore, alleviate many of the problems of the prior art designs.